The present invention is directed to switching mode power supplies (SMPS). More particularly, the invention provides methods and apparatus for reducing the standby power of switching mode power supplies. Merely by way of example, the invention has been applied to reducing the charging current to a pulse width modulation (PWM) and a pulse frequency modulation (PFM) controller in a switching mode power supply. But it would be recognized that the invention has a much broader range of applicability.
Switching mode power supplies (SMPS) have the advantages of smaller size, higher efficiency and larger output power capability, and are widely applied in mobile phone chargers, notebook computer adapters and other fields. In recent years, green power supplies are emphasized, which require higher conversion efficiency and especially lower standby power consumption, because for charger and adapter applications, the power supplies consume only the standby power when they are plugged on the wall socket without connected to the load.
FIG. 1 is a schematic diagram of a conventional primary side regulated (PSR) switching mode power supply system. When the line voltage Vac is applied to the system, the switching signal does not appear (OUT pin remains low) when Vcc voltage is below a threshold voltage (called startup voltage, Vst). This is called the startup phase of the switching mode power supply. In this phase, the Vcc capacitor C1 is charged by the rectified line voltage Vin through startup resistor R1. When Vcc exceeds Vst, the startup phase is finished, the OUT pin of the controller generates switching signals to turn the power transistor 2 on and off to regulate the Vout voltage of the secondary winding. Since the operating current of controller 1 after startup phase is much larger than the current that controller 1 needs at startup phase, the Vcc capacitor C1 must receive energy from the auxiliary winding 4 during the normal operation of the system to supply operating current to controller 1.
In a typical switching mode power supply such as prior art of FIG. 1, the Vcc capacitor C1 is in micro farad range. Smaller value of startup resistor R1 can generate larger charging current for C1, so the startup phase will be faster. But larger current through R1 consumes more energy during normal operation, which increases system standby power.
From the above, it is seen that an improved technique for reducing the standby power of switching mode power supplies is desired.